1. Field of the Invention
The present invention relates to an angle detection device for detecting the angle of rotation of a rotational body such as a swing motor that rotates reciprocally within a prescribed angle range.
2. Description of the Related Art
An optical device such as the one depicted in FIG. 4 that features a slit plate is known as an angle detection device for a swing motor. As depicted in this drawing, the angle detection device 100 is provided with a slit plate 102 in which a helical slit 101 is formed, the slit plate 102 is coaxially fixed to a rotational shaft 103 of the swing motor, and the slit 101 is sandwiched between an LED or other light source 104 on one side and a semiconductor position detection element 105 on the other side. The helical slit 101 is formed across an angle range that includes the rotation angle range of the rotational shaft 103, and when the rotational shaft 103 rotates, the slit portion passing through the irradiation area 106 of illuminating light 104a from the light source 104 moves in the radial direction. Therefore, slit-transmitted light 104b moves in the radial direction in conjunction with the rotation of the rotational shaft 103, and the position in the light-receiving surface 105a of the semiconductor position detection element 105 that is irradiated by the slit-transmitted light moves accordingly. This irradiated position is detected on the basis of the end-to-end output of the semiconductor position detection element 105, and the rotation angle of the rotational shaft 103 is known on this basis. In the rotation angle detection device thus configured, the irradiation area 106 of the illuminating light must be made large enough to contain the range of movement of the helical slit in the radial direction, and the entire irradiation area 106 must be illuminated with a uniform luminance.
A swing motor that with an angle detection device of this configuration is disclosed, for example, in JP-A 2001-339910.
However, the rotation angle detection device that uses a slit plate and a semiconductor position detection element has drawbacks in that the light that is actually used consists solely of the slit-transmitted light 104b that has passed through the slit 101, and the efficiency of light usage is extremely poor.
Drawbacks also exist whereby nonuniformities in irradiation by the light source contribute to a decline in the linearity of the output signal of the semiconductor position detection element, and detection precision is reduced. Specifically, any light source has nonuniform irradiation, so it is difficult to uniformly illuminate the irradiation area 106 with a prescribed width, and the linearity of the output signal is compromised because the intensity of slit-transmitted light and the luminous energy received by the semiconductor position detection element fluctuate according to the slit position when the irradiation area 106 is not uniformly illuminated.
Nonuniform irradiation is particularly severe when an LED or incandescent bulb is used as the light source, and the nonuniform irradiation varies according to the size of current supplied. Therefore, the light source current fluctuates and nonuniform irradiation varies when light compensation is applied in order to made the luminous energy received by the semiconductor position detection element constant, and the linearity of the output signal of the semiconductor position detection element is adversely affected as a result. Nonuniform irradiation also occurs due to variations in ambient temperature when an LED or incandescent bulb is used, which manifests itself as a temperature drift in the output signal, and the linearity of the detection signal is adversely affected.
An optical system may be added and the nonuniform irradiation from the light source improved in order to allow the irradiation area 106 to be uniformly illuminated, but this method is not preferred because the structure of the optical system becomes complicated and the costs increase.
Furthermore, a noise component inherent to semiconductors is included in the output of a semiconductor position detection element, and this limits the resolution as a detector. The intensity of the light source and the S/N ratio of the output signal must be increased in order to increase the resolution.
A swing motor is often used in applications that require high response, such as an optical scanning device or the like, so it is preferred that the inertial moment be made as small as possible. It is therefore preferable that the slit plate attached to the rotational shaft also be made small. However, because the slit must be formed across a prescribed angle range, it is difficult to reduce the size of the slit plate.